1. Field of the Invention
The present invention relates to a muting apparatus for use in a car audio system in which a digital audio signal is transmitted between component units of the audio system.
2. Description of Background Information
On-vehicle audio systems (car audio systems) are generally designed that the power voltage of the system is directly supplied from a storage battery of the vehicle. Consequently, when the vehicle has its engine started, the voltage of the storage battery drops. Such drop in voltage of the battery affects the system in operation, often causing the loudspeaker of the system to produce abnormal sounds or noises. In order to prevent such noise the battery voltage is checked to detect the drop of the voltage. When the drop of the voltage is detected, audio signals are muted in the system.
FIG. 1 shows the structure of a conventional car audio system.
In the drawings, there is shown a component-type car audio system which is constructed of a plurality of functional component units, for example such as source unit 1, a graphic equalizer 2, and an amplifier unit 3.
A player 11 in the source unit 1 reproduces music and the like recorded in compact cassette tapes or compact disks to obtain an analog audio signal which is supplied to the graphic equalizer 2 through an audio signal transmission line S1. The voltage supplied from the battery of the vehicle is supplied to the player 11 and also to a power abnormality detection circuit 12. When the supplied voltage drops to a value lower than a predetermined value, the power abnormality detection circuit 12 issues a power abnormality detection signal to a base of a transistor 13.
FIG. 2 shows an example of a concrete construction of the power abnormality detection circuit 12. In the construction shown in FIG. 2, when the voltage supplied from the battery is larger than a breakdown voltage of a Zener diode TD, i.e., when there is no drop in the voltage of the battery, a transistor Q1 is switched on so that the power abnormality detection signal is not issued. On the other hand, when the voltage supplied from the battery is lower than the breakdown voltage of the Zener diode TD, i.e., when there is a drop of the voltage of the battery, the transistor Q1 is switched off so that the power abnormality detection signal is issued.
The transistor 13 has its collector connected with the audio-signal transmission line S1 and its emitter connected to ground, so that the emitter is at ground potential (hereinafter referred to as the GND potential). The transistor 13 is switched on in accordance with the power abnormality detection signal supplied to its base so that the audio-signal transmission line S1 is fixed at the GND potential.
An equalizer circuit 21 in the graphic equalizer unit 2 adequately controls the analog audio signal in frequency characteristics, which audio signal has been supplied through the analog audio-signal transmission line S1. The thus controlled analog audio signal is then supplied to the amplifier unit 3 through an analog audio-signal transmission line $2. The voltage supplied from the battery of the vehicle is supplied to the equalizer circuit 21 and also to the power abnormality detection circuit 22. This detection circuit 22 issues a power abnormality detection signal to a base of a transistor 23 when the thus supplied voltage drops to a value lower than the predetermined value. The transistor 23 has its collector connected with the audio-signal transmission line S2 and its emitter connected to ground, so that the emitter is at the GND potential. The transistor 23 is switched on in accordance with the power abnormality detection signal supplied to its base so that the audio-signal transmission line S2 is fixed at the GND potential.
An amplifier 31 in the amplifier unit 3 adequately amplifies the analog audio signal which has been supplied through the audio-signal transmission line S2. The thus amplified audio signal is then supplied from the amplifier 31 to an input terminal "a" of a relay switch 33 in which the audio signal received in the input terminal "a" is issued from an output terminal b to a loudspeaker 4. The voltage supplied from the battery of the vehicle is supplied to the amplifier 31 and also to the power abnormality detection circuit 32. This detection circuit 32 issues a power abnormality detection signal to a switch-control terminal c of the relay switch 33 when the supplied voltage drops to a value lower than the predetermined value. When the power abnormality detection signal is issued, the relay switch 33 cuts an electric connection between the input terminal "a" and the output terminal b to prevent the analog audio signal from being supplied to the loudspeaker 4, which analog audio signal is issued from the amplifier 31.
As described above, in the conventional car audio system: the power abnormality is detected in each of the components of the system; and, when the power abnormality is detected, the audio-signal transmission line is fixed at the GND potential in each of the components of the system, so that the audio signal is forced to have a value of 0 level, which makes it possible to accomplish a muting condition in which the loudspeaker is prevented from issuing the abnormal sounds or noises when the voltage drops.
However, in recent years, transmission of the audio signal between audio component units is made through digital signals, which makes it impossible to mute the audio signal by fixing the audio-signal transmission line at the GND potential.